The use of plastics where metal had been used before can be advantageous for several reasons, for example for economy and for the prevention of rust and metal-to-metal contact, but previous attempts to mount plastics parts in openings in parts of cans have not been generally satisfactory because of the difficulty of obtaining a secure connection and a good seal. A high seal integrity is especially necessary for food and carbonated beverage products which will deteriorate if not kept in a hermetically sealed environment.
One known way of mounting a plastics member in an opening in a sheet metal member involves forming the metal member with a curled neck around the opening, and inserting the plastics member in a snap-fit manner into the curled neck. The plastics member has a flange preventing it from passing right through the opening, and a body which is an interference fit in the neck; sometimes an entry bead is also provided, which requires to be forced through the opening and which thereafter helps to secure the plastics member in position. However, the plastics member is required to be resiliently deformable for its fitting into the opening, and so there is a tendency for deformation of the plastics material to occur if the body of the plastics member is later subjected to pressure urging it outwardly of the opening; impaired sealing, and possibly even expulsion of the plastics member from the opening, can therefore result.
It is also known to mould a plastics member in situ in an opening in a sheet metal member, the plastics member being thereby dimensioned so as, at least, initially, to be firmly retained within the opening. However, plastics materials have a "memory", and the plastics member may subsequently shrink slightly due to cold creep of the plastics material, so possibly impairing the seal between the plastics and the metal. Plastics creep may also occur in plastics members which are preformed (rather than formed in situ), again tending to impair the plastics-to-metal seal provided.
Thus there is a need for a method and apparatus capable of reliably sealing a plastics member in an opening in a sheet metal member, particularly in the field of can manufacture.